#Haskell, 85 bytes
My first submission, it will probably be the longest (85 bytes) but hey, you got to start somewhere. In Haskell:
import Numeric;f=('#':).concatMap((flip showHex)"".(15-).fst.head.readHex.(:[])).tail
It's using the same subtract from 15 trick I saw other people use.
I also tried using printf along with the other trick (subtract 8^8 - 1) and it works in ghci but for some reason it doesn't compile:
g=printf "#%06x" .(8^8-1-).fst.head.readHex.tail
If someone could make this work that would be great!
